Characterizing the Relationship between Petrophysical Trends and Diagenetic Cementation Patterns: A Case Study from the Carbonate Arab C Reservoir in the Dukhan Field, State of Qatar

The Upper Jurassic Arab-C reservoir in the Dukhan Field is the product of a stable epicontinental shelf subjected to continuous relative sea level variations, with the preserved sediments representing a complex of syndepositional carbonate lithologies and textures deposited in subtidal, intertidal and sabkha environments. Though the depositional units are spatially correlated within a sequence stratigraphic context, petrophysical observations don’t conform to the primary depositional fabric, and a cross-cutting relationship is observed.

Petrographic analysis highlights sporadic occlusion of primary inter-particle porosity by overgrowth burial-diagenetic calcite cements. Though this form of cementation is observed throughout the reservoir, the prevalence is observed to increase significantly below an inferred free water level (palaeo_FWL), associated with a concomitant reduction in porosity relative to a preserved higher porosity above the contact. By inference, a continuum of diagenetic overprinting has occurred during burial and is directly related to the temporal variation in hydrocarbon charge displacing water within the trap, which regulates both the abundance and aqueous-phase transport potential for calcium and carbonate ions.

The reservoir has a high spatial sampling density of core and petrophysical logs (800m average well spacing) and detailed analysis of the porosity distribution and trends has facilitated the interpretation of structural closure development during burial, hydrocarbon charge and structural tilting which have interacted to produce the current petrophysical spatial configuration. Doming related to the formation of the Qatar Arch has instigated the recent uplift in the southern part of the Dukhan Field, with the tilt levering diagenetically cemented facies from below the palaeo-FWL to a new position above the re-equilibrated current FWL. The palaeo-FWL encloses the preserved high porosity interval, and is spatially coincident with a ring of bitumen occurrence. At ground level, Neogene Hofuf fluvial deposits are uplifted and tilted, and constrain the maximum age for the uplift event.

By virtue of the analysis, process and the recognition of inter-linked structural and diagenetic processes, a unified porosity trend model has been formulated which captures the sum of the individual components from a common generic standpoint, and has helped to increase the predictability of the simulated inter-well porosity variations.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California